1. Field of the Invention
This invention relates to a method of forming plastic molded articles by admixing a blowing material in a plastic raw material and conducting expansion molding, and plastic molded articles formed thereby, and more particularly to a method of forming plastic mold articles which are suitable for obtaining spherical forms, such as baseballs, softballs, golf balls.
2. Background of the Related Art
In molding spherical forms from a synthetic resin (hereinafter referred to as "plastic" or "resin") as a raw material, unevenness of internal temperature distribution inevitably occurs to the spherical forms when they are cooled, irrespective of whether molding is effected by injection or any other molding method, since a spherical form is small in its surface area and thick diametrically. Therefore, it has been very difficult to uniformly obtain a spherical product having an accurate shape. To eliminate this inconvenience, there is known a technique which uses a plastic raw material together with a blowing material, thereby blowing a gas into the resin, for preventing what is called "the phenomenon of sink marks" from occurring during molding. However, even by this expansion molding method, it has been difficult to mold homogeneously-blown spherical forms due to unevenness of temperature distribution in a molten plastic raw material supplied to the inside of a mold. More specifically, the blown state of the resin varies within the mold such that the resin becomes less dense from the outer part toward the central part within the mold. In an extreme case, there is formed a cavity or large hollow space therein, causing a difficulty in molding a spherical form in a uniformly-blown state. If such an unevenly blown or expanded spherical form, e.g. a ball for a ball game, is made in spite of such a difficulty, the ball undergoes unstable movement while flying, which makes it impossible to use the ball for the ball game.
Further, it is a common knowledge that any thread-wound spherical form such as baseballs, softballs, golf balls, and the like should be high in density in their central portions. However, if such articles are molded from a plastic raw material, there is inevitably formed a vacuum foam in the central portion thereof due to the relationship between the cooling of the resin and thermal insulation of the raw material, which makes the density of the central portion markedly lower than that of the outer portion thereof. To eliminate such a drawback, a composite molding technique called "insert molding" has been proposed, in which a core is inserted or placed within a mold before molding a spherical form. Even the insert molding, however, suffers from the following problems: For holding the core within the mold, hold pins are used. It is normally impossible for the hold pins to withstand pressure applied in molding, unless they are thick enough to have a diameter of approximately 6 mm. Accordingly, it is required to use such thick hold pins, which results in holes formed within the molded article by the holding pins for holding the core, and when the hold pins are extracted therefrom, there are vacant portions left in the molded article. If the molded article is spherical, the vacant portions delicately put the gravitational force exerted on the article out of balance, so that if the central portion thereof is formed in an unbalanced state, there is an inevitable inconvenience that the spherical form undergoes unstable movements when flying at a high speed.
Further, there are the following general problems with plastic articles formed by molding: plastic materials mixed with glass fibers for enhancing the strength of molded articles have conventionally been used as reinforcing materials for use in parts of automotive vehicles, motorboats, and the like, and expanded plastic obtained by mixing blowing materials with plastic raw materials are used in other fields related to housings and daily commodities, such as beddings, cushions, imitation leathers, and insulators. However, in the case of the former, i.e., the plastic materials mixed with glass fibers, the hardness of the molded articles is increased while causing roughness of the surface thereof. Moreover, if the fibers become short due to kneading of resin materials by screws, the intended object cannot be attained, making it impossible to obtain soft molded articles. On the other hand, however hard a material is in itself, if it is molded into an expanded material by blowing a gas therein, the resin per se tends to become filament-like to lose its strength. Therefore, the latter, i.e. the expanded articles have been generally considered to be low in tearing strength up to now.